This invention relates to secondary emitter cathodes and more particularly to semiconductor cathodes in the form of a P-N junction which may be made conductive or non-conductive by a pulsed source to provide or not provide, respectively, a secondary electron emission and, in a preferred embodiment, to turn-on or turn-off, respectively, a crossed field amplifier.
Crossed-field amplifier tubes (CFAs) are usually used only in the one or two highest-power stages of an amplifier chain, where their efficiency is significant, and are usually preceded by a medium power traveling wave or klystron tube which provide most of the chain gain. CFAs may be of the reentrant type where the DC electric field between the anode and cathode and the transverse DC magnetic field cause the electrons emitted from the cathode to be accelerated by the electric field and gain velocity, but the greater the velocity, the more the path of the electrons is bent by the magnetic field. As a result, in the absence of an RF field in the interaction space between the cathode and anode, the electrons are bent back to impinge upon the cathode without reaching the anode and, ideally there is no current through the tube. An RF field of the correct frequency will interact with some of these electrons to extract energy from them and cause them to reach the anode and produce an anode current.
In CFAs, it is possible in most cases to use a cold cathode. Even with a "good" vacuum inside the tube, there are sufficient gas molecules present so that some will be ionized when sufficient RF power is provided to the input of the CFA. Some of the free electrons thus produced will be driven back to the cathode. Alternatively, the high RF electric field at the cathode may produce these free electrons by field emission effects from the cathode. The returning electrons will initiate secondary emission from suitable cathode material and full cathode current will very rapidly build up. Under high average power condition, cooling of the cathode may be necessary to prevent overheating. Removal of the applied RF electric field does not cause the tube to immediately cease conducting thereby resulting in oscillation or noise output unless the DC electric field is also removed.
In the prior art, cold secondary emitter cathodes of the non-semiconductor type were used in crossed-field amplifier tubes. These tubes may contain an auxiliary non-emitting control electrode located between the cathode and the anode of the amplifier. A tube operating directly from a DC supply may be turned off with a control voltage pulse (positive with respect to the cathode) applied to the control electrode at the time of removal of the RF drive pulse to collect electrons passing through the drift region and to cause the tube to turn off even though high voltage is still applied. The control electrode forms a segment within a drift region of the periphery of the cylindrical surface of the cathode but insulated from it. A disadvantage of the prior art pulsing technique is that the cut-off .mu., the ratio of the anode voltage to cut-off voltage, is low, approximately 3.0. The cut-off current drawn by the control electrode is approximately 25% of the rated beam current. Cooling of the control electrode is difficult because it is electrically insulated from the cathode which is also cooled to prevent thermionic emission. Also, since a DC-operated control electrode CFA must withstand full DC voltage continuously without breakdown, its peak-power rating cannot be made as great as that of a comparable cathode-pulsed tube.